(1) Field of the Invention
This invention relates to a photocatalytic powder, and a process for producing the powder, which is used as an environmental clarification material for removing malodors, decomposition-removing harmful substances or pollutants in air, effecting drainage or water purification, or killing bacteria or algae in water, which is particularly suitably used in the form such that the powder is incorporated in organic fibers or plastics by kneading or embedding. The invention further relates to a polymer composition comprising the photocatalytic powder, a shaped article made from the composition, and a process for making the shaped article.
(2) Description of the Related Art
For preventing or removing malodors or removing harmful substances in air, a method of absorbing these by an acid or alkali solution or adsorbent has heretofore been widely used in many fields. However, this method has a problem in the treatment of a waste solution or an adsorbent used which may cause secondary pollution. A method of concealing malodors using a fragrance may also be used, however, the odor of fragrance may soak into food, thus, this method has a fear of damages by the odor of fragrance itself (see, for example, Konosuke Nishida, Daihyakka-jiten (Encyclopedia), Vol. 1, p. 136, published by Heibon Sha (1984)).
When titanium oxide is irradiated with light, an electron having a strong reducing action and a positive hole having a strong oxidizing action are generated and a molecular seed coming into contact therewith is decomposed by the oxidation-reduction action. Using such an action, namely, photocatalytic action of titanium oxide, organic solvents dissolved in water, environmental pollutants such as agricultural chemicals and surface active agents, or harmful substances in air or malodors can be decomposition-removed. This method utilizes only titanium oxide and light and can be repeatedly used, and moreover, the resulting reaction product is a harmless carbon dioxide or the like. Furthermore, this method is more free of restriction on the reaction conditions such as temperature, pH, gas atmosphere and toxicity, as compared with the biological treatment using microorganisms, and is advantageous in that those, which are difficult to decompose or remove by the biological treatment, such as organic halogen-containing compounds or organophosphorus compounds, can be easily decomposed and removed.
However, in conventional researches on decomposition and removal of organic materials using photocatalysis of titanium dioxide, a titanium dioxide powder as it is has been used as the photocatalyst (see, for example, A. L. Pruden and D. F. Ollis, Journal of Catalysis, Vol. 82, 404 (1983); H. Hidaka, H. Jou, K. Nohara and J. Zhao, Chemosphere, Vol. 25, 1589 (1992); and Teruaki Kubo, Kenji Harada and Kei-ichi Tanaka, Kogyo Yosui (Industrial Water, No. 379, 12 (1990)). Accordingly, there are difficulties in the handling or use, for example, the used photocatalyst is difficult to recover. Due to this, use of a titanium dioxide photocatalyst has been not easily realized in practice. To overcome this problem, use of a titanium dioxide catalyst by kneading it into fibers or plastics which are easy to handle has been attempted. However, not only the harmful organic materials or environmental pollutants but also the fibers or plastics themselves are readily decomposed by the strong photocatalytic action of titanium dioxide to cause serious deterioration. Thus, use of titanium dioxide photocatalyst by kneading it into fibers or plastics cannot be actually employed.
JP-A-9-239277 (the term "JP-A" as used herein means an "unexamined Japanese patent publication") has proposed a photocatalyst carrier comprising a titanium dioxide particle on which surface an optically inactive compound such as aluminum, silicon or zirconium is island-like supported. This proposed technique of treating the surface of a titanium dioxide particle with an optically inactive compound such as aluminum, silicon or zirconium to support the compound is a technique originally developed in the field of cosmetics or pigments so as to greatly reduce the catalytic activity of titanium dioxide. However, this method suffers from a contradiction such that when the reaction of titanium oxide with fibers or plastics is intended to prevent, the photocatalytic action is in turn greatly reduced at the same time. Furthermore, when the surface-treated titanium dioxide is used as a photocatalyst for a bactericidal or fungicidal material, since fungi scarcely adhere to the photocatalyst, for example, under running water, the photocatalytic effect is difficult to bring out, the efficiency is poor and the durability is inferior.